The present invention relates to a resistor connector employed to restrict the current value of a control circuit, or to detect the resistance value to conduct identification of electrical components. Furthermore, the present invention relates to a manufacturing method of such a resistor connector.
An example of the above resistor connector is shown in FIG. 21. This resistor connector C has a resistor 10 fixed in a manner bridging across a pair of parallel lead frames 1, 1. Said resistor 10 is covered with a mold resin 2 in a state where the terminal portion of each lead frame 1 projects. In general, resistor 10 has a resistor unit 12 provided on a substrate 11. Resistor unit 12 is covered with a glass protection film 13, and each of electrodes 14, 14 is partially lead out from protection film 13. The portion of each electrode 14 leading out from protection film 13 is electrically connected to each lead frame 1 by solder 15. Solder 15 also serves to fasten resistor 10 to lead frame 1.
The site where this resistor connector C is installed is often in a severe environment such as in the neighborhood of the engine compartment of an automobile. Variation in temperature is significant in such an environment. Although resistor connector C exhibits variation in the resistance value depending upon the change in temperature, the variation in the resistance value must be within a tolerable range. The varying range of the resistance value is preferably as small as possible.
In this type of resistor connector C, occurrence of a crack in resistor unit 12 causes change in the resistance value. It is therefore desirable to suppress generation of a crack in resistor unit 12. However, many conventional resistor connectors already have cracks in glass protection film 13. This is considered to be caused by the molding pressure during the mold cover process and the residual strain after the molding process. If such a resistor connector C is exposed to severe change in temperature, thermal stress (thermal shock) occurs due to the difference in the coefficient of thermal expansion between mold resin 2 and resistor 10. This thermal stress causes the crack in protection film 13 to grow to result in a crack in resistor unit 12, as well as increasing the crack in resistor unit 12, when already present. Not only generation of a crack, but the growth of a crack induces a great change in the resistance value. In view of the foregoing, it is required to prevent generation of a crack in protection film 13 as well as in resistor unit 12 during the mold covering process. It is also important to prevent generation of cracks in such regions during the temperature cycle, i.e., during usage.
In view of the above requirements, the present applicant developed the approach to provide a buffer layer between resistor 10 and mold resin 2 (refer to FIG. 2 in the first embodiment of the present invention; wherein silicon resin 20 in FIG. 2 corresponds to this buffer layer), as disclosed in Japanese Patent Laying-Open No. 2000-173803. By virtue of this buffer layer, generation of a crack in resistor 10 during a mold covering process is suppressed to a minimum level. Also, even in the case where there is a severe change in temperature to cause thermal stress in resistor 10 due to the difference in the coefficient of thermal expansion between mold resin 2 and resistor 10, the thermal stress can be absorbed to suppress generation of a crack in resistor 10 during the temperature change to a minimum level.
This resistor connector C has an extremely small variation width of the resistance value during temperature change, and is highly reliable. It was considered that the scattering of the flux volatile components from the solder can be prevented by entirely covering the resistor including the solder by that buffer layer (refer to FIG. 11 of the second embodiment in the present invention).
However, it is to be noted that, at the interface between, for example, the silicon resin forming the buffer layer and the metal lead frame, secure adhesion is obtained only partially. That strength could not be achieved at the entire interface. The volatile component of the solder flux flows out from a portion of the interface to adhere to the terminal portion of the lead frame or enter and contaminate the mold resin. Adhesion to the terminal portion induces connection (contact) failure to result in a defective product. Intrusion in the mold resin will result in a defective product from the standpoint of appearance. As a result, the above-described resistor connector had an extremely low yield.
An object of the present invention is to prevent scattering of solder flux volatile components.
To achieve the above object, the present invention regulates the flow of solder to the terminal portion as one approach. Accordingly, the solder will not arrive at the terminal portion. Connection failure caused by adhesion of solder is eliminated.
As another approach, the flux volatile component is forced to volatilize before the mold covering process. By the forced volatilization, the amount of flux volatile component in the solder during the mold covering process is reduced. Accordingly, the amount that adheres to the terminal portion is reduced.
In view of the above object, a resistor connector of the present invention includes a pair of parallel lead frames, a resistor with a pair of electrodes, and a mold resin. Each of the above lead frames includes a terminal portion and an electrode connection portion. The pair of electrodes are electrically connected to respective electrode connection portions of the pair of lead frames. The mold resin covers the resistor so as to expose the terminal portion. The lead frame includes solder flow inhibit means between the electrode connection portion and the terminal portion for inhibiting flow of the solder towards the terminal portion.
If the cover by the mold resin is thin in the above structure, the flux volatile component may break through the thin mold resin cover due to the mold pressure and the like to flow to the surface of the cover during the covering process. To cope with the situation, a wall or trench is preferably formed so as to enclose the entire perimeter of the electrode connection portion (the solder land region). By setting the enclosed region to the least required level, the required amount of solder can be minimized. Accordingly, the absolute scattering amount of the flux volatile component can be reduced. Also, the amount of flow towards the surface of the cover can be reduced in accordance with the lower scattering amount. Furthermore, by selecting that region as the solder position (the place to mount the resistor), the solder will be gathered at the predetermined position in the region by solder fusion even if the solder print position protrudes from that region at the time of reflow. This travel causes the resistor to be shifted to the predetermined position to be attached accurately. In other words, self alignment is effected. This wall or trench is also advantageous in suppressing scattering of the flux volatile component towards the direction along the interface between the resin mold cover and the lead frame. Possible materials for the formation of this wall include resist printing, chromium coating, nickel coating, and the like that have poor wettability with respect to solder.
Preferably in the above invention, a buffer layer is provided at the interface between the resistor and the mold resin. As the material of the buffer layer, silicon resin (rubber), urethane type resin (rubber) and the like can be cited from the standpoint of heat resistance, adherence, hardness, spreadability, and the like. The buffer layer also serves to prevent scattering of the flux volatile component in addition to the role to protect the resistor, as described above. The buffer layer is preferably minimized in amount, as long as there is no problem. For example, the buffer layer may be provided only on the top plane of the resistor among the interface between the resistor and the mold resin. Alternatively, the buffer layer may be formed at the gap between the companion lead frames of the resistor in addition thereto. This is because, for the purpose of maintaining the durability, only the direct thermal stress towards the portion that requires only the minimum support is to be alleviated such as the glass protection film of the chip resistor, for example.
To achieve the above object, a method of manufacturing a resistor connector of the present invention includes a connection step of providing, by printing, solder on the electrode connection portion of the pair of parallel lead frames to electrically connect a resistor to the electrode connection portion via the solder, and a covering step of covering the resistor with mold resin so as to expose a terminal portion of the lead frame. Furthermore, the manufacturing method includes a thermal treating process of exposing the flux solid component of the solder at high temperature to carbonize sufficiently and for sufficient volatilization of the volatile component.
Volatilization of the flux volatile component can be effected by, when the resistor is to be electrically connected to the lead frame through reflow of solder as the connection step, setting the temperature, the period of time, or both the temperature and the period of time of the reflow higher, longer, or both higher and longer than the normal case. Also, a thermal treating step for carbonization and volatilization of flux can be additionally carried out after the electrical connection step by reflow of solder.
Preferably in the present invention, the solder is lead-free (solder not including lead). As lead-free solder, Snxe2x80x94Ag eutectic solder, Anxe2x80x94Ag Cu solder, and the like can be cited as examples. For example, Anxe2x80x94Ag eutectic solder has a melting point of approximately 221xc2x0 C., which is higher than the melting point of approximately 183xc2x0 C. of Snxe2x80x94Pb eutectic solder. Therefore, when Anxe2x80x94Ag eutectic solder is employed, for example, the reflow temperature inevitably becomes higher to promote flux volatilization.
Preferably in the present invention, the lead frame is entirely coated with nickel, and the electrode connection portion (solder portion) is coated with silver. This provides higher heat resistance in response to the higher reflow temperature. Furthermore, affinity with the lead-free solder can be improved.
Preferably in the present invention, the lead frame has a wall or trench formed. This is more effective since the flow of solder can be inhibited. In the present invention, a step of forming a buffer layer to cover a predetermined portion of the resistor is added prior to the covering step. This is preferable from the standpoint of preventing generation of a crack in the resistor since the buffer layer is located between the resistor and the mold resin.